Their forecasts, underwritten by the National Oceanic and Atmospheric Administration, call for a mid-summer "hypoxic" zone measuring 1.46 cubic miles, whith another 0.26 to 0.38 cubic miles of water containing essentially no oxygen for fish and shellfish to breathe. Last year's mid-summer low-oxygen zone was roughly the same size.

Fish become stressed when dissolved oxygen levels in water decline to hypoxic levels, and they can suffocate in water devoid of the life-sustaining gas.

"Dead zones" are caused by nutrient pollution getting into the water from sewage, farm runoff and even air pollution, causing algae blooms that consume the oxygen in the water when the microscopic plants die and decay. But weather plays a big role, with rain, winds and temperatures affecting the amount of pollution getting into the water and how it affects water quality.

Donald Boesch, president of the UM Center for Environmental Science, said the bay's smaller dead zone this year is due in part to a dry spring, which washed less pollution than usual into the Chesapeake.

The US Geological Survey estimates 36,600 metric tons of nutrients got into the bay from the Susquehanna and Potomac rivers, the bay's two largest tributaries. That's about 30 percent below the average seen since 1990, the agency reports.

While less rainfall helped reduce runoff, Boesch said he believes the long-running bay cleanup also has had a positive influence on the dead zone. Measurements of pollution, when adjusted for variations in weather, appear to be declining, he noted.

"It's nothing to say we've solved the problem," he added. "There are some encouraging signs, but I’m sure much more needs to be done to reduce the loads."

While the bay's outlook is rosier, not so for the Gulf of Mexico, which scientists say appears headed in the other direction, with a larger-than-usual fish-stressing expanse of water expected to form that could reach the size of New Jersey.

Last year, with a drought limiting runoff in the Missisippi River basin, the Gulf's dead zone was the fourth smallest ever measured. This year, though, with spring flooding across the Midwest, nutrients washing into Gulf tributaries appear to be about 16 percent above the long-term average.

Boesch said that while the Chesapeake cleanup effort appears to be slowing pollution, there is no similar campaign on the Gulf.